Wide assortments of implantable medical devices are presently known and commercially available. These implantable medical devices include a variety of implantable cardiac devices. For example, implantable pulse generators (IPGs) are a type of cardiac device that is generally used to elevate the heart rate that is beating too slowly. This type of device is sometimes referred to as a bradycardia device or a pacemaker. Another type of implantable cardiac device is an implantable cardiac defibrillator (ICD). This type of device, often referred to as a tachycardia device, is generally used to provide burst pacing pulses or a defibrillation shock to the heart when the heart is beating too fast or goes into fibrillation. Another type of device is a cardiac resynchronization device used to treat heart failure.
Each of these types of implantable cardiac devices includes a sensor or sensors to monitor the patient's cardiac system to facilitate determination of when and what action to take. For example, many previous implantable cardiac devices have relied upon electrical sensors extended into the right ventricle of the heart. These electrical sensors measure the electrocardiogram (ECG) signal in the heart to determine how well the heart is functioning, and to determine what, if any, action the implantable cardiac device needs to take. Unfortunately, extending a lead into the heart, or attaching a lead to the outside of the heart is a relatively invasive procedure and is thus not desirable for all patients. Without a sensor into the heart it can be problematic to effectively monitor the patient's cardiovascular status. Specifically, the presence of spurious electrical signals caused by muscle movement and other factors can interfere with attempts at cardiac monitoring using sensors that are not extended into or attached to the outside of the heart itself.
Thus, there remains a need for additional implantable sensing techniques for monitoring a patient's cardiovascular status.